Small portable night vision system

ABSTRACT

Night vision systems that are compact in size due to one or more of the design of the optical system and light detector module.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/217,158, filed on Dec. 12, 2018, which is a divisional application of U.S. application Ser. No. 15/017,763, filed on Feb. 8, 2016, which claims the benefit of priority of U.S. Provisional Application No. 62/113,656, filed on Feb. 9, 2015, the entire contents of which application(s) are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to night vision systems, and more particularly, but not exclusively, to night vision systems that are compact in size due to the design of one or more of the optical system and the light detector module.

BACKGROUND OF THE INVENTION

Existing night vision systems, which include optics, a detector, a display, and power supply, are bulky, because state-of-the-art devices are optimized for resolution and sensitivity, but not size and weight. Three types of night vision technologies are currently deployed. A first one makes use of an image intensifier, which amplifies light ˜30,000 to 1,000,000 times, usually at the wavelength range of 0.4 to 1 micron. Since moonlight or starlight is usually present at night, a scene can be made visible provided there is enough amplification with high signal-to-noise ratio. The image intensifier includes a photocathode, a single or multiple microchannel plate, and a phosphor screen that emits green light, to which the human eye is most sensitive. Light is absorbed by the photocathode and converted to electrons, which are amplified by the microchannel plate. The electrons are then accelerated to the phosphor screen at high voltage (˜600-900V), resulting in the generation of more light. In a typical night vision system, the image intensifier is sandwiched between the imaging optics and eyepiece, resulting in a bulky tubular structure. The image intensifier can also be coupled directly to a CCD detector by using intermediate optics or a fiber bundle.

A second night vision technology is application of active illumination in near infrared wavelengths (˜0.7 to 1 micron) and detection using a conventional silicon based focal plane array such as a CCD or CMOS. The technique is used extensively indoors. For outdoor applications, the range of view is limited by the intensity and directionality of the illumination source. A third technology is thermal imaging. Objects at ambient temperature emit long wavelength infrared radiation at 7.5 to 15 microns. The radiation can be detected using InSb, InGaAs, HgCdTe, a quantum well infrared photodetector (QWIP), or microbolometer focal plane arrays. In many applications, the thermal image is combined with a visible image acquired using conventional silicon focal plane array to provide a thermal map of an object or scene.

SUMMARY OF THE INVENTION

In one of its aspects the present invention provides low weight and size, portable night vision systems with direct line of sight, utilizing freeform optics and compact integration of detectors and displays. For example, one particularly useful optical design of the present invention incorporates wedge-shaped lenses having freeform surfaces in both the collection optics and display optics. In this regard, exemplary designs of the present invention may include a wedge-shaped, freeform objective as the collection optics and a wedge-shaped, freeform eyepiece as the display optics. The collection optics of the present invention may capture an image of a scene for amplification by an image intensifier. The image intensifier may electrically communicate through appropriate electronics, such as a computer, with the microdisplay for displaying the intensified image. As used herein, the term “electrically communicate” is defined to include both wired and wireless communication and combinations thereof. The appropriate electronics, such as a computer, may be a physically separate unit from the microdisplay and image intensifier, or may be incorporated as part of one or more of the image intensifier and microdisplay. The display optics of the present invention may be configured to form an image of the microdisplay at an eye pupil for viewing by a user.

For instance, in one exemplary configuration the present invention may provide a compact night vision system including an imaging device configured to receive light from a scene and a microdisplay disposed in electrical communication with the imaging device for generating an image to be viewed by a user. The image may be relayed to the user by a wedge-shaped, freeform eyepiece which has a first freeform surface positioned to receive light from the microdisplay, a second freeform surface configured to receive the light transmitted into the body of the eyepiece from the first freeform surface and configured to reflect the received light at the second surface, and a third freeform surface configured to receive the light reflected by the second freeform surface and configured to transmit the light out of the eyepiece. The first, second, and third freeform surfaces of the eyepiece may be positioned to provide the wedge-shape to the eyepiece.

In one desirable exemplary configuration, the imaging device may be a camera, such as a thermal imaging camera. Alternatively, the imaging device may include a wedge-shaped, freeform objective and an image intensifier positioned to receive light transmitted out of the objective. The image intensifier may be disposed in electrical communication with the microdisplay. The freeform objective may include a first freeform surface configured to receive light from a scene, a second freeform surface configured to receive the light transmitted into the body of the objective from the first freeform surface of the objective and configured to reflect the received light at the second surface of the objective, and a third freeform surface configured to receive the light reflected by the second freeform surface of the objective and configured to transmit the light out of the objective for delivery to the image intensifier. The first, second and third freeform surfaces of the objective may be positioned to provide the wedge-shape to the objective.

In another of its aspects, the present invention may provide a light detector module, which may be particularly compact, and therefore well-suited to night vision systems. The detector module may include a photocathode, a microchannel plate (MCP), a lens array, and a detector array, FIGS. 4A-5B. The compact configuration may be provided by a lens array coated with phosphor materials. Light generated by phosphor material may be collected by the microlenses of the array and focused onto and detected by respective individual pixels of the focal plane array.

In one desirable configuration, the light detector module may include, from a first end to a second end: a photocathode layer, a microchannel plate, a lens array, and a detector array where each element of the detector array is disposed in registration with a respective lens of the lens array. The lens array may include a plurality of microlenses coated with a phosphor layer and may include a barrier structure disposed between two adjacent microlenses of the microlens array. In addition, the lens array may include an Einzel lens array and the detector may include a Faraday cup array, a delta-doped CCD, an electrometer array, or a focal plane array.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following detailed description of exemplary embodiments of the present invention may be further understood when read in conjunction with the appended drawings, in which:

FIG. 1-3 schematically illustrate exemplary configurations of optical designs of compact imaging night vision systems in accordance with the present invention;

FIGS. 4A-4C schematically illustrate an exemplary configuration of a light detector module in accordance with the present invention comprising a phosphor coated microlens array and focal plane array;

FIG. 5A schematically illustrates a further exemplary configuration of a light detector module in accordance with the present invention comprising an Einzel lens array and Faraday cup array; and

FIG. 5B schematically illustrates an exemplary configuration of a light detector module in accordance with the present invention comprising an Einzel lens array and a delta-doped charge-coupled device.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, wherein like elements are numbered alike throughout, the present invention may provide particularly compact and portable night vision systems, where the compact and portable features are due in part to one or more of the design of the optical system 100 and/or light detector module 400. For example, in one of its aspects the present invention may provide an optical system layout that is compact due to the inclusion of freeform optics, such as a wedge-shaped, freeform lens 110, 120 (e.g., a prism-lens), FIGS. 1-3, for example. The freeform optics may be the collection optics 120, the display optics 110, or both.

In exemplary configurations of the present invention as illustrated in FIGS. 1-2, a freeform objective 120 may be provided in the form of a wedge-shaped, freeform prism-lens. Likewise a freeform eyepiece 110 may be provided in the form of a wedge-shaped, freeform prism-lens. Each of the freeform objective 120 and freeform eyepiece 110 may define separate optical paths, such as a night vision capture path 10 and night vision image display path 12, respectively. The night vision capture path 10 may include the freeform objective 120, an objective stop, and an image intensifier 20, while the night vision image display path may include the freeform eyepiece 110 prism and a microdisplay 30. The image intensifier 20 may operate in the 0.4 to 1 micron range and may receive and intensify an image from the capture path 10, and then electronically communicate the intensified image to the microdisplay 30 for use in the night vision image display path 12. Both the freeform objective 120 and the freeform eyepiece 110 may be formed by multiple freeform optical surfaces S1-S3, S4-S6.

In the capture path 10, photons from a scene may pass through the objective stop and reach the detector (e.g., the image intensifier 20) through consecutive refraction and reflections by the prism-like objective 120. A principal advantage of the exemplary configurations of FIGS. 1-2 is the folding of the optical path inside a solid prism such as the freeform eyepiece 110 and/or freeform objective 120, which enables the detector 20 and/or microdisplay 30 to be packaged to the side. (In contrast, state-of-the art night vision goggles suffer from the large volume due to the co-linear restriction of conventional rotationally symmetric optics.) In the display path 12, the captured image may be processed in real time and displayed on the microdisplay 30. Light from the microdisplay 30 may thus propagate through the freeform eyepiece 110, via multiple consecutive refractions and reflections, and reach the eye pupil.

Turning to FIG. 1 in more detail, the exemplary freeform objective design shown therein has a field of view of 40 degrees and system F/number of 1.1, which achieves a fast objective 120 needed for night vision use. The display design shown in FIG. 1 offers an equivalent 40-degree field-of-view (FOV), a 25 mm eye clearance, and a 10 mm eye pupil. The overall volume of the optical system is approximately 35 mm thick, 40 mm high, and 25 mm wide, which is substantially more compact than the state-of-the-art night vision goggles. Example of a suitable commercially available image intensifiers include Image Intensifier DS001 (Photek Limited, East Sussex, UK) and iStar 334T (Andor Technology Ltd, Belfast, UK) that combines an image intensifier and CCD.

FIG. 2 schematically illustrates an additional configuration of a compact and portable night vision system 200 in accordance with the present invention which is similar in certain respects to the configuration of FIG. 1. In particular the freeform objective 120, freeform eyepiece 110, microdisplay 30, and image intensifier 20 may all be the same in both the configurations of FIGS. 1 and 2. However, in the configuration of FIG. 2, the freeform objective 120 is inverted relative to the freeform eyepiece 110 to permit the freeform objective 120 and freeform eyepiece 110 to be positioned more closely together. For example, whereas the freeform eyepiece 110 and freeform objective 120 in the configuration of FIG. 1 are separated by 35 mm, in the configuration of FIG. 2 the freeform eyepiece 110 and freeform objective 120 are separated by 30 mm.

In yet another exemplary configuration in accordance with the present invention, as schematically illustrated in FIG. 3, an imaging device, such as a thermal camera module 50, operating in a 7.5 to 15 micron range may replace the freeform objective 120 using the configuration of FIGS. 1-2. The camera module 50 may collect information from a scene and electrically communicate it to the microdisplay 30. An example of a suitable commercially available thermal camera module is the Quark 2 Uncooled Thermal Imaging Camera Core (FLIR, Wilsonville, Oreg.) with 6.3 mm lens and dimensions of 22×22×35 mm. The position of the thermal camera module 50 may be set to provide real time imaging close to the direct line-of-sight of the eye. Reduction of parallax between measured image and image seen by the naked eye is important for night vision goggle application.

An optical prescription of the exemplary freeform eyepiece 110 of FIGS. 1-3 is listed in Tables 1-3 for surfaces S1, S2, and S3, respectively. Of the three optical surfaces in the freeform eyepiece 110, surface S1 is an anamorphic aspheric surface (AAS). The sag of an AAS surface is defined by

${z = {\frac{{c_{x}x^{2}} + {c_{y}y^{2}}}{1 + \sqrt{1 - {\left( {1 + K_{x}} \right)c_{x}^{2}x^{2}} - {\left( {1 + K_{y}} \right)c_{y}^{2}y^{2}}}} + {{AR}\left\{ {{\left( {1 - {AP}} \right)x^{2}} + {\left( {1 + {AP}} \right)y^{2}}} \right\}^{2}} + {BR\left\{ {{\left( {1 - {BP}} \right)x^{2}} + {\left( {1 + {BP}} \right)y^{2}}} \right\}^{3}} + {{CR}\left\{ {{\left( {1 - {CP}} \right)x^{2}} + {\left( {1 + {CP}} \right)y^{2}}} \right\}^{4}} + {DR\left\{ {{\left( {1 - {DP}} \right)x^{2}} + {\left( {1 + {DP}} \right)y^{2}}} \right\}^{5}}}},$

where z is the sag of the freeform surface measured along the z-axis of a local x, y, z coordinate system, c_(x) and c_(y) are the vertex curvature in x and y axes, respectively, K_(x) and K_(y) are the conic constant in x and y axes, respectively, AR, BR, CR and DR are the rotationally symmetric portion of the 4th, 6th, 8th, and 10th order deformation from the conic, AP, BP, CP, and DP are the non-rotationally symmetric components of the 4th, 6th, 8th, and 10th order deformation from the conic.

Surface S2 of the freeform eyepiece 110 may be an XY polynomial surface defined by:

${z = {\frac{c\left( {x^{2} + y^{2}} \right)}{1 + \sqrt{1 - {\left( {1 + k} \right){c^{2}\left( {x^{2} + y^{2}} \right)}}}} + {\sum\limits_{j = 2}^{66}{C_{j}x^{m}y^{n}}}}},{j = {{\left\lbrack {\left( {m + n} \right)^{2} + m + {3n}} \right\rbrack/2} + 1}}$

where z is the sag of the freeform surface measured along the z-axis of a local x, y, z coordinate system, c is the vertex curvature (CUY), k is the conic constant, and Cj is the coefficient for x^(m)y^(n).

Surface S3 may be an aspheric surface with a rotationally symmetric kinoform diffractive optical element, with the sag of the aspheric surface defined by:

${z = {\frac{cr^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)c^{2}r^{2}}}} + {Ar^{4}} + {Br^{6}} + {Cr^{8}} + {Dr^{10}} + {Er^{12}} + {Fr^{14}} + {Gr^{16}} + {Hr^{18}} + {Jr^{20}}}},$

where z is the sag of the surface measured along the z-axis of a local x, y, z coordinate system, c is the vertex curvature, k is the conic constant, A through J are the 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th, and 20th order deformation coefficients, respectively.

TABLE 1 Optical surface prescription of surface S1 of the freeform eyepiece 110 X Curvature (c_(x))  −1.348215E−02 Y Curvature (c_(y))   2.004523E−03 Y Conic Constant (K_(Y))   0.998125E+01 4th Order Symmetric Coefficient (AR)  −3.9067945E−06 6th Order Symmetric Coefficient (BR)  −9.5768964E−17 8th Order Symmetric Coefficient (CR)  −2.8799927E−15 10th Order Symmetric Coefficient (DR)  −8.7077963E−16 X Conic Constant (K_(X))  −1.5687534E+01 4th Order Asymmetric Coefficient (AP)  −3.2949463E−01 6th Order Asymmetric Coefficient (BP)  −2.0405356E+02 8th Order Asymmetric Coefficient (CP)  −8.0782710E+00 10th Order Asymmetric Coefficient (DP) −2.72019184E−01

TABLE 3 Optical surface prescription of surface S3 of the freeform eyepiece 110 Y Radius −1.5000000000E+01 Conic Constant (K) −8.1715030467E+00 4th Order Coefficient (A) −3.5999478362E−05 6th Order Coefficient (B) 4.1811989405E−07 8th Order Coefficient (C) −2.0382499300E−09 10th Order Coefficient (D) 3.7498678418E−12 Diffraction Order 1 Construction Wavelength (nm) 550 R**2 (HCO C1) −3.2332326174E−03 R**4 (HCO C2) 4.1482610496E−05 R**6 (HCO C3) −4.2185152895E−07 R**8 (HCO C4) 1.8253428127E−09 R**10 (HCO C5) −2.7615741244E−12

TABLE 2 Optical surface prescription of surface S2 of the freeform eyepiece 110 Y Curvature −1.26056882299E−02  Y Radius −7.93292664201E+01  Conic Constant (SCO K | C1) 1.99429650209E+00 X (SCO X | C2) 0.00000000000E+00 Y (SCO Y | C3) 0.00000000000E+00 X**2 (SCO X2 | C4) −2.8963611697E−03 X * Y (SCO XY | C5) 0.00000000000E+00 Y**2 (SCO Y2 | C6) 5.13151841830E−04 X**3 (SCO Y3 | C7) 0.00000000000E+00 X**2 * Y (SCO X2Y | C8) −1.6871196613E−05 X Y**2 (SCO XY2 | C9) 0.00000000000E+00 Y**3 (SCO Y3 | C10) −3.9628025988E−05 X**4 (SCO X4 | C11) 5.63763951591E−07 X**3 * Y (SCO X3Y | C12) 0.00000000000E+00 X**2 * Y**2 (SCO X2Y2 | C13) −5.1451820404E−07 X * Y**3 (SCO XY3 | C14) 0.00000000000E+00 Y**4 (SCO Y4 | C15) 1.52902584933E−06 X**5 (SCO X5 | C16) 0.00000000000E+00 X**4 * Y (SCO X4Y | C17) 2.30036831137E−08 X**3 * Y**2 (SCO X3Y2 | C18) 0.00000000000E+00 X**2 * Y**3 (SCO X2Y3 | C19) 3.82949206634E−08 X * Y**4 (SCO XY4 | C20) 0.00000000000E+00 Y**5 (SCO Y5 | C21) −9.3057372440E−08 X**6 (SCO X6 | C22) −2.3473886032E−09 X**5 * Y (SCO X5Y | C23) 0.00000000000E+00 X**4 * Y**2 (SCO X4Y2 | C24) −2.4682522624E−09 X**3 * Y**3 (SCO X3Y3 | C25) 0.00000000000E+00 X**2 * Y**4 (SCO X2Y4 | C26) −3.5764311583E−09 X * Y**5 (SCO XY5 | C27) 0.00000000000E+00 Y**6 (SCO Y6 | C28) −4.3636504848E−09 X**7 (SCO X7 | C29) 0.00000000000E+00 X**6 * Y (SCO X6Y | C30) −1.8300632292E−10 X**5 * Y**2 (SCO X5Y2 | C31) 0.00000000000E+00 X**4 * Y**3 (SCO X4Y3 | C32) −1.0237987168E−10 X**3 * Y**4 (SCO X3Y4 | C33)  0.0000000000E+00 X**2 * Y**5 (SCO S2Y5 | C34)  2.0693559836E−10 X * Y**6 (SCO XY6 | C35)  0.0000000000E+00 Y**7 (SCO Y7 | C36)  2.1203645386E−10 X**8 (SCO X8 | C37)  2.6638311623E−12 X**7 * Y (SCO X7Y | C38)  0.0000000000E+00 X**6 * Y**2 (SCO X6Y2 | C39)  4.2552541871E−12 X**5 * Y**3 (SCO X5Y3 | C40)  0.0000000000E+00 X**4 * Y**4 (SCO X4Y4 | C41)  −4.101261981E−12 X**3 * Y**5 (SCO X3Y5 | C42)  0.0000000000E+00 X**2 * Y**6 (SCO X2Y6 | C43)  3.9696325158E−12 X * Y**7 (SCO XY7 | C44)  0.0000000000E+00 Y**8 (SCO Y8 | C45)  1.7421792489E−11 X**9 (SCO X9 | C46)  0.0000000000E+00 X**8 * Y (SCO X8Y | C47)  2.8416565461E−13 X**7 * Y**2 (SCO X7Y2 | C48)  0.0000000000E+00 X**6 * Y**3 (SCO X6Y3 | C49)  7.7200373777E−13 X**5 * Y**4 (SCO X5Y4 | C50)  0.0000000000E+00 X**4 * Y**5 (SCO X4Y5 | C51)  −6.188783932E−13 X**3 * Y**6 (SCO X3Y6 | C52)  0.0000000000E+00 X**2 * Y**7 (SCO X2Y7 | C53)  1.7935251959E−14 X * Y**8 (SCO XY8 | C54)  0.0000000000E+00 Y**9 (SCO Y9 | C55)  −1.391093985E−13 X**10 (SCO X10 | C56) −2.6923251198E−15 X**9 * Y (SCO X9Y | C57) 0.00000000000E+00 X**8 * Y**2 (SCO X8Y2 | C58) −1.5546422781E−14 X**7 * Y**3 (SCO X7Y3 | C59) 0.00000000000E+00 X**6 * Y**4 (SCO X6Y4 | C60) −1.0384073178E−14 X**5 * Y**5 (SCO X5Y5 | C61)  0.0000000000E+00 X**4 * Y**6 (SCO X4Y6 | C62)  3.8750232363E−14 X**3 * Y**7 (SCO X3Y7 | C63)  0.0000000000E+00 X**2 * Y**8 (SCO X2Y8 | C64)  −3.094245370E−14 X * Y**9 (SCO XY9 | C65)  0.000000000E+00 Y**10 (SCO Y10 | C66)  −3.15607172E−14

For the freeform objective 120 of FIGS. 1-2, one or more of the surfaces (S4, S5, S6) in the design layout may utilize a type of freeform surface, Tables 4-5. Each of the surfaces of the freeform objective 120 was modeled as an “XY Poly” type. The term “XY Poly” refers to a surface which may be represented by the equation

$z = {\frac{cr^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} + {\sum\limits_{j = 2}^{66}{C_{j}x^{m}y^{n}}}}$ ${j = {\frac{\left( {m + n} \right)^{2} + m + {3n}}{2} + 1}},$

where z is the sag of the freeform surface measured along the z-axis of a local x, y, z coordinate system, c is the vertex curvature (CUY), r is the radial distance, k is the conic constant, and C_(j) is the coefficient for x^(m)y^(n). The optical prescriptions for these surfaces (S4 through S6) are listed in Table 4, while the surface decenters with respect to the global origin which coincides with the center of the eye box are listed in Table 5.

TABLE 4 Optical surface prescription of the freeform objective 120 S4 S5 S6 Y Radius −525.2 −65 −50 k 1.324 −0.52 2.25 X**2 −0.0015 −0.002156 −1.387e−5 Y**2 −0.00025 −1.55e−6 −1.821e−5 X**2 * Y 1.5e−5 0.00012 0.000174

TABLE 5 Optical surface positions and orientations of the freeform objective of Table 4 with respect to the center of the objective stop Orientation of Origin of surface reference the surface Rotation X (mm) Y (mm) Z (mm) about X-axis θ (°) S4 0 0 3 5.5 S5 0 0 12 −20.5 S6 0 18 9 65 Detector* 0 −1 4 2 Note: In Table 5, the detector coordinates are defined relative to S6, instead of the objective stop.

In another of its aspects, the present invention provides a light detector module 400, which may be particularly compact, and therefore well-suited to night vision systems. Light detector modules of the present invention are expected to provide space-saving advantage over conventional systems, which typically use a fiber bundle array or relay optics to couple an image intensifier 20 to a CCD. The use of a fiber bundle array or relay optics requires additional separation between the image intensifier and CCD, leading to an increased and undesirable size.

Exemplary light detector modules of the present invention can effect night vision imaging by measurement of visible and near infrared photons using an image intensifier in combination with a silicon based focal plane array or delta-doped CCD. In particular, in an exemplary configuration in accordance with the present invention, the light detector module 400 may include a photocathode 410, a microchannel plate (MCP) 420, a lens array 430, and a detector array 440, FIGS. 4A-5B. The process of light amplification starts when light is absorbed in the photocathode 410 and electrons are emitted. The electrons accelerate to the microchannel plate 420 which generates more electrons. The lens array 430 may be coated with a phosphor material layer 432, FIG. 4B, may be biased relative to the microchannel plate 420, so that electrons are accelerated toward the phosphor coated lens array 430 to generate light. The lens array 420 may be disposed on a substrate 435. Light generated by phosphor layer 432 may be collected by microlenses 434 of the array 430 and focused onto and detected by respective individual pixels of the focal plane array 440.

Turning to the photocathode 410 in more detail, the photocathode 410 converts incident light into electrons by the photoelectric effect. The quality of the photocathode 410 may be characterized by the quantum efficiency (QE), which is defined to be the percentage of incident photons that are converted to electrons. QE is generally wavelength dependent. Depending on the required spectral sensitivity, different photocathode materials can be used. Examples of suitable photocathode materials for use in the present invention include alkali, multi-alkali alloys (lithium, sodium, potassium, rubidium, cesium, antimony, silver) and semiconductor (GaAs, GaAsP, InGaAs, Cs—Te).

The MCP 420 may be positioned to receive electrons created by the photocathode 410. The MCP 420 then amplifies the electron signal, usually by >10⁴ times. The MCP 420 may include a thin metal oxide coating to prevent ion feedback to the photocathode 410. Suitable MCPs for use in light modules of the present invention include LongLife′ MCP (Photonis, Sturbridge, Mass.) and F1551 MCP (Hamamatsu Photonics, Bridgewater N.J.).

After the electrons received by the MCP 420 are amplified, the resulting electrons may be accelerated by a constant voltage and subsequently collide with a phosphor material. In one exemplary configuration of the present invention, the phosphor material may be provided as a phosphor layer 432 on the microlenses 434, FIG. 4B. The transparent substrate 435 of the microlens array 430 may be biased by a positive voltage to attract the electrons. A portion of the electron kinetic energy may be converted to heat and the remaining energy absorbed by the phosphor to excite the phosphor atoms. The excited phosphor returns to the ground state by emission of light. Phosphor materials typically emit green light and may be made of rare earth oxides or halides, such as P20=Y₂O₂S:Eu, P43=Gd₂O₂S:Tb, P45=Y₂O₂S:Tb, P46=Y₃Al₅O₁₂: Ce, P47=Y₂SiO₅:Ce, for example. The type of phosphor may be chosen to match the peak sensitivity of the detector array and the required frame rate. High frame rates require a phosphor with short fluorescence decay time. The phosphor may be deposited by the settling process, the slurry process, the dusting process, or the phototacky process. Information about different typed of phosphor and their processing can be found in William M. Yen, et al., “Practical Applications of Phosphors,” CRC Press 2007. After the electrons are converted back to photons through the excitation and de-excitation of the phosphor layer 432, the photon signal may be detected by the detector array, such as a focal plane array 440 which may be a CCD or CMOS detector array, for example, FIG. 4A.

Turning to the lens array 430 more particularly, in one particularly useful configuration of the present invention, the lens array 430 may include a plurality of microlenses 434, each of which may be coated with a phosphor layer 432, FIG. 4B. The microlens array 430 may be utilized to couple light to the CCD to reduce the separation of the image intensifier (microchannel plate 420 and photocathode layer 410) and the focal plane array 440, leading to an overall more compact design. The spacing, or pitch, between microlenses 434 may be selected to correlate to the spacing or pitch between light sensing pixels in the focal plane array 440, FIG. 4C. The center of each microlens 434 of the array 430 may align with the center of the active area of a respective pixel of the focal plane array 440. By providing a microlens array 430, and matching the microlens pitch to that of the focal plane array 440, the lens array 430 may increase the optical fill factor by collecting photons proximate to each pixel, and delivering such photons into the active area of each pixel of the focal plane array 440.

The lens array 430 may be made of glass or polymer using techniques such as resist reflow, gray scale lithography, embossing, and casting. The material of the lens array 430 may desirably have low optical loss at the emission wavelength of the phosphor. A barrier structure 436 may also be provided on the lens array 430, which may include a conducting material, such as metal or semiconductor. The barrier structure 436 may remove excess charge build up on the phosphor layer 432 and separate the phosphor layer 432 into different spatial regions, such that light emitted from each spatial region is collected mainly into the single lens 434 adjacent the spatial region and respective pixel of the focal plane array 440. The barrier structure 436 may reduce pixel cross talk, by preventing light emitted from neighboring phosphor spatial regions from reaching the same pixel. The barrier structure 436 may be fabricated by conventional microfabrication techniques such as photolithography, sputtering, and etching.

In one exemplary configuration of the detector array of the present invention, the lens array 430 may be fabricated directly on top of the focal plane array 440, with a separate transparent substrate 435, phosphor layer 432, and a barrier structure 436 mounted on top. In another configuration, the detector array 440 may contain the barrier structure 436, the phosphor layer 432, and the lens array 430 directly fabricated on top of the focal plane array 440, which may be fabricated using conventional microfabrication techniques.

In another exemplary configuration of a light detector module in accordance with the present invention, the detector module 500 may include a photocathode 510, a microchannel plate 520, and a micro Faraday cup array 540, FIG. 5A. (An example of a Faraday cup array suitable for use in the present invention may be found in U.S. Pat. No. 8,866,080 and references disclosed therein.) In this configuration, electrons coming from the microchannel plate 520 are detected by the array 540 of Faraday cups. An electrode 560, such as a mesh electrode, may be placed between the microchannel plate 520 and the Faraday cup array 540 to bias the electrons, and each Faraday cup may be connected to a transimpedance amplifier which converts the current signal to voltage signal. Further, an array of Einzel lenses 530 may optionally be placed between the microchannel plate 520 and the Faraday cup array 540 to refocus the electrons. (An example of an Einzel lens array suitable for use in the present invention may be found in US Patent Application Publication 2013/0341526. See also Tomono et al., “Fabrication of Einzel Lens Array with One-Mask RIE Process For Electron Micro-Optics,” Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009.) Similar to the configuration of FIG. 4A, the size and pitch of the lenses of the Einzel array 530 may be matched to the size and pitch of the array 540 of Faraday cups.

In yet another exemplary configuration of a light detector module in accordance with the present invention, the detector module 600 may include a photocathode 610, a microchannel plate 620, and a delta-doped CCD 630, FIG. 5B. An example of a delta-doped charge-coupled device suitable for use in the present invention is FastCamera 300 (FastVision LLC, Nashua, N.H.). Electrons from the microchannel plate 620 may be detected directly by each pixel at the delta-doped CCD 630. An electrode and/or array of Einzel lenses 640 may be placed between the microchannel plate 620 and the delta-doped CCD 630 to bias or refocus the electrons. The size and pitch of the lenses of the Einzel array 640 may be matched to the size and pitch of the delta-doped CCD 630. Alternatively a delta-doped silicon p-i-n diode array may be used instead of the delta-doped CCD 630 to directly detect and image low energy electrons. (See Nikzad, et al., “Direct detection and imaging of low-energy electrons with delta-doped charge-coupled devices,” Applied Physics Letters, Volume 73, p. 3417, 1998. Nikzad et al., “Direct detection of 0.1-20 keV electrons with delta doped, fully depleted, high purity silicon p-i-n diode array,” Applied Physics Letters, Volume 89, p. 182114, 2006). Other charge detectors, such as a MEMS electrometer array, may also be used.

Moreover, any of the light detector modules disclosed herein, such as those illustrated in FIGS. 4A-5B may be used as the image intensifier 20 in the designs of FIGS. 1-3.

These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.

A number of patent and non-patent publications are cited herein; the entire disclosure of each of these publications is incorporated herein by reference. 

What is claimed is:
 1. A compact night vision system, comprising: a wedge-shaped, freeform objective including: a. a first freeform surface configured to receive light from a scene, b. a second freeform surface configured to receive the light transmitted into the body of the objective from the first freeform surface of the objective and configured to reflect the received light at the second surface of the objective, c. a third freeform surface configured to receive the light reflected by the second freeform surface of the objective and configured to transmit the light out of the objective, wherein the first, second and third freeform surfaces of the objective are positioned to provide the wedge-shape to the objective; a light detector module positioned to receive light from the scene transmitted out of the objective, the light detector module comprising a lens array comprising a plurality of microlenses each having a respective curved surface, each curved surface conformally coated with a phosphor layer, the phosphor layer structured to convert electrons to light which is focused by the microlenses; and a microdisplay disposed in electrical communication with the imaging device for generating an image to be viewed by a user.
 2. The compact night vision system according to claim 1, wherein the lens array comprises a barrier structure disposed between two adjacent microlenses of the microlens array.
 3. The compact night vision system according to claim 1, wherein the lens array comprises an Einzel lens array.
 4. The compact night vision system according to claim 1, wherein the light detector module comprises a Faraday cup array, a delta-doped CCD, or an electrometer array.
 5. The compact night vision system according to claim 1, wherein the light detector module comprises a focal plane array.
 6. The compact night vision system according to claim 1, wherein the wedge-shaped, freeform objective has a field of view of at least 30 degrees.
 7. The compact night vision system according to claim 1, wherein the wedge-shaped, freeform objective has an f-number of 2 or less.
 8. The compact night vision system according claim 1, wherein the light detector module comprises, from a first end to a second end, a photocathode layer, a microchannel plate, and the microlens array. 